Solid-state batteries are referred in the state of the art as electrochemical cells containing at least an anode, a cathode and a solid electrolyte. These cells offer a number of advantages over electrochemical cells containing a liquid electrolyte, especially improved safety features.
The secondary batteries with the highest energy density used today use lithium, wherein lithium ions are the active charge carrying species. Advanced secondary lithium battery systems require electrolytes with specific properties such as wide electrochemical stability windows, high mechanical strength alone or when imbibing a porous separator, and/or chemical inertness or non-solvency towards the electrode materials at any stage of charge or discharge. It is also desirable that electrolytes are non-flammable, non-volatile, do not leak and are non-toxic, making them safer both in use and after disposal. In pursuit of such materials, several classes of electrolytes have been studied as replacement for conventional liquid electrolytes, either of inorganic or organic nature: polymers, polymer composites, hybrids, gels, ionic liquids and ceramics.
Typical materials used for the manufacture of solid electrolytes can be inorganic matrices, such as β-alumina and Nasicon, sulfide glasses in the SiS2+Li2S+LiI system or simple Lithium halide with enhanced grain boundaries defect induced by nanoparticles oxides, like silicon dioxide. All these are brittle materials where the volume changes inevitable during operation induce stress and possible cracks in the electrolytes. In order to have electrolytes compliant with volumes changes, the use of organic polymeric matrices is preferred. Typical examples include polyethylene oxide, polypropylene oxide or polyethyleneimine and their copolymers. These materials are used in combination with a suitable lithium salt, such as lithium tetrafluoroborate (LiBF4) and lithium bis(triflruoromethane sulfonyl imide [Li(CF3SO2)2N] referred in the following as LiTFSI.
Conductivity levels sufficient for battery operation (10−5-10−3 S·cm−1) are only obtained above room temperature, from 50 to 80° C.
The polymers containing the (CH2CH2O)n repeat unit are the most conductive, and the polymers containing this unit have been most studied. The tendency for segments with n≥15 to crystallize require to function above the melting point, as only the amorphous phase is conducting, whether this sequence is in copolymers of the random type or block, or of comb type. However, at the temperature of operation, the polymers have insufficient mechanical properties to act as an electrolyte and separator in a battery. This is especially true when lower Mw α-ωmethyl-end-capped polyoxyethene unit with 4≤n≤20 (known ad PGDME) are used as additive to plasticize the membrane. Crosslinking is usually necessary to improve the mechanical resistance, which in turn decrease the thermal motion of the chains, hence the conductivity. The cross-linking process is usually slow and diminishes the speed of battery production.
The main disadvantage of all these polymer electrolytes is the ambipolar conductivity. When a current is applied, both the anions and cations are mobile, then ≈⅓ of the current through the electrolyte is transported by the cation and ⅔ by the anion. This aspect is quantified by the transport number t+ defined as t+=σcation/σcation+σanion=Dcation/Dcation+Danion, σ and D being the conductivity and diffusion of each charges species. In most battery electrode systems, only cations react at the electrodes, so eventually the electroneutrality results in an accumulation of salt in the vicinity of the anode and salt depletion close to the cathode. Both over-concentrated and depleted electrolyte have a far lower conductivity, thus the polarization of the cell increases markedly with a reduction in power capability.
Some attempts have been proposed in order to overcome these problems. For example, U.S. Pat. No. 5,569,560 describes the use of an anion complexing agent comprising polyamines with the strong electron-withdrawing unit CF3SO2 attached to slow-down the anions, thus allowing the lithium cations to carry a larger fraction of the current in an electrochemical cell. The effect on the transport number t+ is however minimal. Recently, solvent-free, hybrid electrolytes based on nanoscale organic/silica hybrid materials (NOHMs) have been prepared with lithium salts [Nugent, J. L. et al., Adv. Mater., 2010, 22, 3677; Lu, Y. et al., J. Mater. Chem., 2012, 22, 4066]. Such electrolytes have uniformly dispersed nanoparticle cores covalently to which polyethylene glycol (PEG) chains are covalently bonded. These electrolytes are self-suspended and provides homogeneous fluids where the PEG oligomers simultaneously serve as the suspending medium for the nanoparticle cores and as ion-conducting network for lithium ion transport.
WO2010/083041 also discloses hybrid electrolytes based on NOHMs comprising a polymeric corona attached to an inorganic nanoparticle core, being the polymeric corona doped with lithium salts.
Schaefer, J. L. et al. (J. Mater. Chem., 2011, 21, 10094) also describes hybrid electrolytes based on SiO2 nanoparticles covalently bonded to a dense brush of oligo-PEG chains, doped with a lithium salt, in particular lithium bis(trifluromethanesulfone imide). This electrolyte is prepared in polyethylene glycol dimethyl ether (PEGDME) which provides an excellent ion conductivity. However, the anion of the lithium salt freely moves through the electrolyte and ⅔ of the current is carried by anions, thus generating a high concentration polarization, and therefore an internal resistance and voltage loss.
In all these three last examples, the fact that the free salt is dissolved in the grafted PEG parts of these nano composites means that the transport number t+ is <<1, having as result the same concentration polarization during battery operation.
On the other hand, recent investigation is focused on the development of sodium-ion secondary batteries in which sodium ion is employed in place of lithium ion. The use of sodium as the electrochemical vector for batteries is becoming increasingly popular, as sodium is much more abundant than lithium, and for large-scale applications, like electrical grid storage, it becomes mandatory. However, sodium insertion electrodes undergo large volume changes during operation, and besides, the non-compliant solid electrolyte interface at the electrolyte/electrode surface is much less favorable for sodium. This suggests again the use of polymer electrolytes which are compliant for volume changes and far more stable than conventional carbonate solvents. However, fewer studies than for Li have been devoted to polymer Na-ion batteries. Sodium batteries electrolytes have the same requirement as for lithium, to have the highest possible transport number t+≈1 to avoid concentration polarization.
In this sense, Kumar, D. (J. Power Sources, 2010, 195, 5101-5108) discloses sodium ion conducting, gel polymer electrolyte nanocomposites based on poly(methyl methacrylate) and dispersed with unfunctionalized silica nanoparticles. However, only a slight enhancement in the sodium ion transport is observed due to the dispersion of silica nanoparticles in the gel system.
Kumar (Solid State Ionics, 2010, 181, 416-423) also describes other sodium ion conducting gel polymer electrolyte which comprises a solution of sodium triflate (NaCF3SO3) in an ionic liquid 1-ethyl-3-methyl imidazolium trifluoro-methane sulfonate, immobilized in poly(vinylidene fluoride-co-hexafluoropropylene). Similarly, the anions needed to compensate the charge of the organic cation and of sodium have a far higher concentration and mobility than that of the latter ion.
In view of that, there is still a need to develop lithium and sodium secondary batteries comprising solid electrolytes with improved mechanical properties and ionic conductivity selective to Li+ or Na+ cations. A t+ of ≈1 is, in addition to avoid concentration polarization, the best strategy to avoid the growth of dendrites for Li or Na metal electrodes, which have intrinsically higher energy densities than Li-ion and Na-ion systems.